The present invention relates generally to the field of thermally sprayed coatings applied onto the interior surface of cooking vessels and specifically to the use of thermally sprayed means of electric arc spraying or flame spraying methods to apply a layer of material onto the interior surface of a cooking vessel. This is done to improve the characteristics of the interior surface of the cooking vessel. It is typically done to prepare the cooking surface of the cooking vessel for receiving a non-stick or lubricative coating layer.
The present invention specifically relates to a method for coating a substrate with a very hard surface having a non-stick coating and a resulting cooking utensil. Additionally, the hard surface applied as disclosed herein is substantially chemically inactive where it is applied.
Application of non-stick coatings to cooking vessels is well-known. Adhesion of non-stick coatings (for example non-stock coatings such as the TEFLON brand non-stick coating), which are applied to a substrate metal, is improved by first flame spraying metal droplets onto the substrate layer to give a rough surface for the non-stick coating to adhere to. However, this coating typically must be thin. This is because that, while a thick coating of the non-stick material will increase the non-stickability of the interior surface of the cooking vessel, a thick coating will reduce the scratch resistance of the interior surface of the cooking vessel. Thus, a relatively thin coating is desired to achieve maximum scratch resistance. Accordingly, there is a competition between thin and thick coatings. Thin coatings of non-stick material are desired to enhance durability, while thick coatings are desired to enhance the non-stick ability of the cooking utensil. Since many of the utensils used in cooking are made of metal and are substantially harder than the non-stick coating it is generally desirable to have a non-stick coating which is durable enough to withstand the abrasion of cooking utensils while retaining good non-stick characteristics.
Typically, the way to take advantage of the positive characteristics of both thick and thin coatings is to apply the droplets which form the rough surface of the substrate layer in a manner which presents peaks and valleys. The peaks being the tops of the metal droplets and the valleys being the spaces located therebetween. In this manner a thick layer of non-stick or lubricative material fills the valleys while a thin layer covers the tops of the peaks. Because the peaks are very close together, relative to the size of the cooking utensil used (e.g. a spatula, a fork, a knife, etc.), the cooking utensil glides on the tops of the peaks. Accordingly, because the layer of material is very thin at the top of the peaks, the hardness of metal material used to form the roughened surface enhances the scratch or scuff resistance of the non-stick coating. Consequently, because enhanced scratch resistance is desired, the metal used to form the peaks must be substantially hard so that it can resist the abrasion of the cooking utensil. Thus, a hard metal, typically stainless steel, is chosen as the metal that is to be flame sprayed onto the substrate to form the roughened intermediate layer surface.
However, an additional problem is that the non-stick coating can suffer breakdown due to corrosion between the substrate metal of the cooking vessel and the thermally sprayed intermediate layer (galvanic corrosion) or as the result of cooking high acid content food (e.g., stewed tomatoes) at relatively high cooking temperatures (electrolytic corrosion). Accordingly, metals that are subject to corrosion in combination with a thermally sprayed intermediate layer are not considered suitable. Unfortunately, it is very difficult to prevent the aforementioned type of corrosion from occurring because the metal applied by flame spraying or similar technique is almost always different from the metal which forms the substrate. Even when similar metals are used there may be corrosion because foundry grade metals., e.g. 443-1 aluminum alloy, are not the pure metal but contain other metals or impurities. Two pure metals could be used but the cost of each piece produced by such a process would be commercially substantial.
It is the objective of the present invention to produce a cooking vessel having an intermediate layer which is substantially hard and substantially chemically inactive when applied to the substrate layer of a cooking vessel so that the resulting cooking vessel when coated with a non-stick or lubricative material will have enhanced scratch resistance and be substantially resistant to, if not impervious to, galvanic and electrolytic corrosion.
The present invention is believed to be most suitably used with aluminum cookware but it will also work with stainless steel cookware. The present invention produces cookware having increased durability and improved resistance to corrosion. Consequently, the layer of material produced by the present invention is able to withstand the scraping of cooking utensils and to avoid the blistering caused by corrosion which can occur under the non-stick surface due to the chemical interaction between the intermediate or mechanically durable layer and base layer of the cooking vessel.
The present invention is thus a unique and simple method, and a product produced by that method, for application of a thermally sprayed layer onto a substrate of a cooking vessel for formation of a layer having improved durability as well as improved resistance to corrosion. The inventors know of no prior art which either teaches or discloses the present invention. As noted above, the inventors believe that the present invention is best used with aluminum pans but it may be used with any substrate that could be used to make a cooking vessel.